1. Field of the Invention
The invention relates in general to an apparatus for driving a light tube and the method thereof, and more particularly to an apparatus for driving a cold cathode florescent light (CCFL) tube and the method thereof.
2. Description of the Related Art
With the advantages of being small in size and light in weight, the liquid crystal display has become the mainstream product in display market. As the consumers are requesting a higher standard of display quality, high luminance and high contrast are two important factors when it comes to the selection of a display. A transparent liquid crystal display (LCD) uses a cold cathode fluorescent light (CCFL) tube as the backlight source. The CCFL tube is an important factor in determining the luminance and contrast parameters of a transparent LCD.
The CCFL driving circuit of a tube and the driving method have great influences on the luminance efficiency and lifespan of the CCFL tube. Referring to FIG. 1A, a conventional CCFL driving circuit is shown. The conventional CCFL driving circuit 100 includes NOR gates NO1˜NO5, buffers 125 and 135, switches Q1 and Q2, a transformer T, capacitors C1˜C3, and a resistor R for driving a CCFL tube 150.
Referring also to FIG. 1B, a timing diagram inside the driving circuit 100 is shown. The driving circuit 100 receives at the input end ip a squared wave S1 that has a fixed duty cycle and outputs a squared wave S2 that is delayed by the capacitor C1 and the resistor R for one delay period. NOR gates NO1 and NO2 perform a logic operation on squared waves S1 and S2 for outputting an operation value M1, which is the sum of S1 and S2, i.e., M1=S1+S2. NOR gates NO3 and NO4 perform a logic operation on squared waves S1 and S2 for outputting an operation value M2, which is the product of S1 and S2, i.e., M1=S1·S2. Operation values M1 and M2 control switches Q1 and Q2 via buffers 125 and 135, respectively. When operation value M2 is at a low level, the switch Q2 made of PMOS is switched on and the capacitor C2 is being charged. On the other hand, when operation value M2 is at a high level, the switch Q1 made of NMOS is switched on and the capacitor C2 is being discharged. By alternately switching Q1 and Q2 on and off, the primary voltage Vin of the transformer T is made a squared wave; by changing the duty cycle of the inputted squared wave S1, the duty cycle of the primary voltage Vin is changed responsively.
FIG. 1C is a diagram of the primary voltage and the secondary voltage of the driving circuit 100. Referring to FIG. 1A and FIG. 1C at the same time, the luminance of a light tube 150 is changed by changing the duty cycle of the primary voltage Vin. The secondary current lout, which changes in response to the primary voltage, is of a sine wave due to the capacitor C3 and other stray capacitances. When the duty cycle of the primary voltage Vin becomes larger, the secondary current lout thus becomes stronger, intensifying the luminance of the light tube 150 as shown in period T2. However, when the duty cycle of the primary voltage Vin is not 50%, the secondary current lout is asymmetric, that is, the magnitudes of the positive peak value and the negative peak value are not the same. When the duty cycle of the primary voltage Vin is 50% as shown in period T1 and T3, the secondary current lout is symmetric; When the duty cycle of the primary voltage is not 50% as shown in period T2, the secondary current lout is asymmetric. When a current with poor symmetry is applied to the CCFL tube, an evenly distributed luminance of the tube cannot be achieved; moreover, the lifespan and the reliability of the tube are reduced.